Support for the OAuth2 version of ZoningAPI

**The OAuth2 version of ZoningAPI will be deployed to zoning.us.  This gem will be used in an OAuth2 version of ZoningCheck - the Rails conversion - which will be deployed to zoningclearance.com**

* An Access Token is included in the HTTP headers of each request. Since this is sensitive, using the https:// protocol is highly recommended.
* Connection to the Zoning API required a:
    * `client_id` and `client_secret` found in the Zoning API admin panel on a per-app basis.
    * `username` and `password`, which are the credentials for the user that owns the app.

*Error handling when fetching the access token*

* Connection errors from Faraday or the OAuth2 client gem will be passed through as a Zoning::ConnectionFailedError with appropriate status code and response messages.
* All other errors are captured as Zoning::ConnectionFailedErrors of type: "500: Internal Server Error"

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Also, some initial specs have been added around the error handling functionality.